Novel Torque Ripple Minimization Control for 25Mw Variable Speed Drive System Fed by Multilevel Voltage Source Inverter

NOVEL TORQUE RIPPLE MINIMIZATION CONTROL FOR 25MW VARIABLE SPEED DRIVE SYSTEM FED BY MULTILEVEL VOLTAGE SOURCE INVERTER

by Masahiko Tsukakoshi Drive Systems Department

Mostafa Al Mamun Drive Systems Department

Kazunori Hashimura Motor and Drive Engineering Department

Hiromi Hosoda Drive Systems Department Toshiba Mitsubishi-Electric Industrial Systems Corporation Tokyo, Japan

Junichi Sakaguchi General Manager Research Institute of Technology Innovation & Strategy Chiyoda Corporation Yokohama, Japan and Lazhar Ben-Brahim Professor, Electrical Engineering Department Qatar University Doha, Qatar

generations, and application of artificial neural networks to Masahiko Tsukakoshi joined the Drive power systems. He was also involved in the promotion of Systems Department of Toshiba in 1995. He renewable energy, a COE research project of the Ministry of transferred to the same section of Toshiba Education, Japan. Mitsubishi-Electric Industrial Systems Dr. Al Mamun obtained his Diploma degree (Electrical Corporation (TMEIC), in Tokyo, Japan, in Engineering, 2001) from Ibaraki National College of Technology, 2005. His current research project concerns and B.S. and M.S. degrees (Electrical and Electronics Engineering, industrial applications, for example, hot-strip 2003, 2005) from Tokyo University of Agriculture and Technology. mills and compressors of oil and gas plants. He received his Ph.D. degree (Electronics and Information His research interests are the effective Engineering, 2008) from the same university. He is a member of use of large inverter systems and control JSER of Japan and IEEE of the US. algorithms of drive systems. Mr. Tsukakoshi received B.S. and M.S. degrees (Electrical Engineering, 1993, 1995) from Meiji University, Kawasaki. He is a Kazunori Hashimura is a Project member of the Institute of Electrical Engineering of Japan (IEEJ). Engineering and Management Specialist at Toshiba Mitsubishi-Electric Industrial Systems Corporation, in Tokyo, Japan. He Mostafa Al Mamun joined the Drive is currently managing motor and drive Systems Department of Toshiba Mitsubishi- projects in various industries and applications Electric Industrial Systems Corporation, in in the US, Europe, and Australia. The projects Tokyo, Japan, in 2008. His current research include those in oil and gas industries, projects concern power electronics, especially such as the motors and drives on offshore the development of drive systems for platforms. He is also involved in the general industry and power system analysis. development and testing programs for the new large motor-drive Dr. Al Mamun’s research interests are system for the company. environmental energy engineering, wind Mr. Hashimura received his B.S. and M.S. degrees (Mechanical power, load forecasting, distributed power Engineering, 1995, 1997) from Georgia Institute of Technology. 193 194 PROCEEDINGS OF THE THIRTY-NINTH TURBOMACHINERY SYMPOSIUM • 2010

INTRODUCTION Hiromi Hosoda is a Chief Engineer of LNG is in great demand globally because it is a clean fuel that is Toshiba Mitsubishi-Electric Industrial Systems friendly to the environment. To obtain LNG, the natural gas is chilled Corporation Drive System department, in to 162ЊC to produce a clear liquid that occupies up to 600 times less Tokyo, Japan. He joined the Department of ! space than the corresponding gas. To achieve the necessary cryogenic Power Electronics of Toshiba Corporation, Japan, where he has been engaged in the temperatures, refrigerating turbocompressors are traditionally driven development of motor drives. Now, he is by industrial heavy-duty gas turbines (GTs). Besides their low working as a project leader of the large efficiency, GT need regular maintenance. Furthermore, the necessary VSI drives development. shutdown periods and the unscheduled outages interrupt the LNG Mr. Hosoda received a B.E. degree production and reduce LNG plant productivity. As electrical drives (Electrical Engineering, 1974) from Shizuoka University. such as VSDs are maintenance free and more efficient than GT, efforts are being made by major LNG plant operators, contractors and manufacturers to develop VSDs suitable for LNG compressors. Junichi Sakaguchi joined Chiyoda On the other hand, VSDs have been used in various industries Corporation, Japan, in 1974 and became such as steel and paper mills. In the megawatt capacity ranges, a General Manager in the Mechanical these industries prefer multilevel voltage source inverter (VSI) over Engineering Department in 1996. He the load commutated inverter (LCI) as a power converter for VSD was promoted to Senior General Manager applications. VSIs are preferred due to their lower harmonics, of the Detail Engineering Division in better power factor, and smaller torque ripples at the motor side. 1999. Mr. Sakaguchi continued as an These same features make the VSI fed VSD systems the most executive officer for the departments of attractive solution for driving LNG plant compressors. A new Corporate Planning (2001-2003) and control method is proposed to reduce the harmonics and the torque Technology and Engineering (2003-2005). ripples of a VSI-based VSD drive system. At present he is a Fellow and General Manager in the From previous experience, the installation of a VSI-based drive Research Institute of Technology Innovation & Strategy, in system for a large capacity compressor of an LNG plant led to Yokohama, Japan. several technical issues related to ripples and torsional vibrations in Mr. Sakaguchi received B.S. and M.S. degrees (Mechanical a centrifugal LNG compressor train with a gearbox. Kita, et al. Engineering, 1972, 1974) from the University of Tokyo. (2007), reported that the torsional vibration was transferred to the lateral vibration at the gear mesh. Based on the knowledge of previous coupling failure of a compressor driven by a VSD fed by Lazhar Ben-Brahim is currently a a three-level inverter, the following two methods were implemented Professor at the Electrical Engineering to solve the problem: Department of Qatar University, in Doha, • Synchronized pulse width modulation control for the output Qatar, and Industrial Electronics Chair frequency and for RasGas Company (LNG plant). From 1991 to 1997, he was with Toshiba • V/F constant control (Shimakawa and Kojo, 2007). Corporation, where he was engaged in research and development of power The compressor train has been operating properly after the electronics and electric drive systems. He implementation of the above-mentioned two methods. was Head of the Department of Industrial The objective of this study was to improve the new techniques Technology from 1998 to 2005. His research interests include and to apply them to a five-level large capacity inverter instead of control applications, motor drives, instrumentation, sensors, and a three-level inverter, which resulted in even higher performances. power electronics. An improved control method, based on a fixed pulse pattern, was Dr. Lazhar Ben-Brahim received his B.S. and M.S. degrees also applied to further improve the waveform of the five-level (Electrical Engineering, 1985, 1986) from the University of Tunis, inverter. The effectiveness of the improvements was validated by Tunisia. He received his Ph.D. degree (Electrical and Computer torque ripple measurement during the motor combined experimental Engineering, 1991) from Yokohama National University, in test. The authors built a 25 MW motor drive system and evaluated Yokohama, Japan. He is a senior member of IEEE. the system in a back-to-back test using a 7.2 kV 30 MVA VSI bank along with a 25 MW synchronous motor (SM). A power recovery system with a synchronous generator (SG) and a regenerative PWM ABSTRACT inverter were used to load the high power SM. The relationship between the VSI output voltage pulse pattern and the torque ripple Continuous improvements in the power rating and switching was examined. The effectiveness of the implemented control characteristics of power semiconductor devices have enabled the method was also experimentally verified. use of power electronics converters in high power variable speed drives (VSDs). These multimegawatt drives are needed for driving DRIVE SYSTEM APPLICATION BACKGROUND large capacity compressors in liquefied natural gas (LNG) plants. However, the generated harmonics and their associated torque Stable speed and torque control is essential for a large ripples may result in serious drawbacks in the application of adjustable-speed motor-driven compressor train. In order to VSDs in the oil and gas industry. The torque ripples may lead achieve the stable control, however, highly advanced control to torsional vibrations that may in turn cause damage to the techniques are required. In this section, will be introduced the load-motor coupling. To overcome these drawbacks, a new speed influence of and issues involved with motor speed control of an control technique, which is based on a synchronized pulse width adjustable speed drive that were experienced. modulation (PWM) control method, is proposed. A 25 MW five Fuel Gas Compressor System level VSD system was developed to verify the new approach using two experimental tests, namely, back-to-back and full load tests. and Analysis of a Coupling Failure The tests validated the feasibility of the proposed method in A schematic diagram of the studied fuel gas compressor system reducing the torsional vibration. is shown in Figure 1. A 13.65 MW induction motor was driven by NOVEL TORQUE RIPPLE MINIMIZATION CONTROL FOR 25MW VARIABLE 195 SPEED DRIVE SYSTEM FED BY MULTILEVEL VOLTAGE SOURCE INVERTER a PWM converter and a PWM inverter. A gear was used to increase Compared with an LCI, a VSI has the newest high frequency the speed. Two compressors (low pressure compressor and high switching device and can be controlled independently of the power pressure compressor) were driven at the increased speed. grid voltage or motor voltage. The advantages of a VSI over LCI are smooth startup and small motor torque ripple. Figure 4 shows a single line diagram of a VSI.

Figure 1. Fuel Gas Compressor System.

It was found that the speed feedback showed continuous vibration with a measured vibration frequency of 14 Hz, which is Figure 4. Single Line Diagram of a Voltage Source Inverter. the resonant frequency of the compressor system. The resonance was caused by torsional and torque ripple onsite. As a result, the LP VSIs offer considerable advantages over the widely used LCI compressor coupling was broken after 2000 hours of operation. drives considering the following cases: Figure 2 shows an actual photograph of the coupling failure Because a thyristor has to be turned off by the aid of the power between a gear and an LP compressor (Kocur and Corcoran, 2008). • grid or motor voltage, the grid and motor characteristics require special design to realize proper LCI operation. Therefore, a specially designed motor and additional power compensating equipment are necessary. Also, a VSI can be designed independently of the power grid and motor conditions due to the high controllability of self turned-off devices. A VSI drive can be applied to a standard motor without any modified grid. • An LCI has a low power factor with lower order harmonic current. Therefore, power factor correction equipment and Figure 2. Photographs of Coupling Failure of an LP Compressor. harmonic filter capacitors are necessary to maintain the power (Kocur and Corcoran, 2008) system quality. The power factor correction equipment and harmonic filters have to be carefully designed to avoid parallel and The PWM control of the inverter was not synchronized to the series power resonance. However, a highly reliable power system output frequency, resulting in a side band wave, which was the can be obtained from a VSI even in the case of a weak power cause of the torque ripple. Torsional vibration of the compressor supply system, because no capacitor is required. shaft caused the speed signal ripple. The vector control was based on the speed signal and it amplified the speed vibration. The PRINCIPLE OF THREE-LEVEL mechanical system of the compressor was relatively weak INVERTER AND FIVE-LEVEL INVERTER compared with that of a rolling mill system and the coupling was broken with a small torque ripple in several months of operation. In a two-level inverter, which is commonly used for low voltage The authors overcame this problem by analyzing the failure and (LV) alternating current (AC) drives, the output voltage waveform applying two methods, that is: is produced by using PWM with two voltage levels. In a three-level inverter, which is commonly used for higher power VSI, the output • A synchronized carrier with the output frequency and voltage and the current waveforms are improved due to the greater number of voltage levels. The total harmonic distortion (THD) is V/F constant control. • also reduced compared with two-level inverters. The efficiency of three-level inverters at full load is also higher than that of two-level The distortion in the motor control with the feedback function inverters, which means better energy handling of the system. A affected the stable operation of the compressor train. The unstable better efficiency at rated power also means a smaller heat sink and control caused by the improved motor control indicates accurate better reliability. The efficiency of three-level inverters at small vector control and converter phase control in the compressor train power is also improved (Ikonen, et al., 2005). Figure 5(a) and (b) with the small damping involve some risk. On the other hand, the show the main circuit configurations of two-level and three-level synchronized control in the PWM inverter is an effective method inverters, respectively. for the large adjustable speed motor in the compressor train. According to the analysis, it was verified that the V/F control for the synchronized PWM inverter efficiently controlled the motor used in the compressor train in which the load fluctuated gradually. The motor driven compressor train could be successfully stabilized by this motor control method (Shimakawa and Kojo, 2007). VSI Versus Conventional LCI Drive The basic configuration of an LCI is shown in Figure 3. The LCI is a conventional drive system using thyristors and has a low switching frequency device. It is controlled by aid of the grid voltage or motor voltage. LCI has higher grid harmonics and larger motor torque ripple.

Figure 5. Development of Main Circuit Technology.

Figure 5(c) shows a VSI with an even higher number of voltage level and a five-level inverter with gate commutated turn-off Figure 3. Single Line Diagram of a Load Commutated Inverter. thyristor (GCT). The diode converter portion has three layers of 12 196 PROCEEDINGS OF THE THIRTY-NINTH TURBOMACHINERY SYMPOSIUM • 2010 phases diode rectifier, which is built without any fuses for a compact structure. The input circuit breaker protects the diodes from short circuit by using a current sensor device at the input of the rectifier. The five-level inverter output voltage and current waveforms are more sinusoidal than the waveforms of the three-level inverter and the obtained THD is lower. An LCI has commonly been used for large capacity compressor applications in the oil and gas industry. However, five-level technology can increase the voltage level and yield large capacity VSI drives, which are more suitable for driving the compressors of LNG plants (Tsukakoshi, et al., 2009). ADVANTAGES OF FIXED PULSE PATTERN Improved control by employing a fixed pulse pattern is introduced in this section. Unlike the sinusoidal PWM and space vector PWM techniques, where the pulses are controlled by the current Figure 6. Output Voltages of Five-Level Switching Circuit. controller and changed in every cycle, in the fixed pulse pattern technique, the same pulse pattern is output in every cycle. This Figures 7 and 8 show a comparison of the harmonic frequency pulse pattern is described below. components in the VSI output power with the asynchronous pulse With the conventional PWM control, the controller changes the pattern and the fixed pulse pattern, respectively, with respect to the pulse width at every instant to regulate the VSI current output and motor frequency. keep it sinusoidal. Then the pulse pattern changes cycle by cycle and asynchronous to the output frequency. In the VSI, the inverter outputs a voltage according to the pulse pattern, which means that the output voltage also changes its shape cycle by cycle. The frequency spectrum of such an asynchronous voltage shape generally contains harmonic components in a wide frequency band, including low frequency components that may cause problems with mechanical oscillation. In order to avoid the above risk, the authors developed the controller to apply the fixed pulse pattern. The principle of the pattern generation is well-known and has been described in the literature when VSIs were first developed. However, the algorithm Figure 7. Characteristics of the Harmonic Frequency Components required very high control performance and resources that could in the VSI Output Power with the Asynchronous Pulse Pattern with not be realized by the commercial products available at that time. Respect to the Motor Frequency. These days, owing to the development of CPUs and memories, fixed pulse pattern control can be implemented in practical controllers and can be used online in real time. According to a pulse control theory, one can make a pulse pattern that eliminates low order harmonics. By repeating such a pulse pattern every cycle, the frequency spectrum of the output voltage should not contain low frequency components. The pulse patterns are calculated offline by computer taking the output voltage value as the parameter. Then, the calculation results are installed in the memory of the VSI controller. The controller chooses appropriate pulse pattern data according to the voltage required to drive the motor and outputs the pattern synchronized with the frequency. Figure 6 shows an example of a fixed pulse pattern. Figures 6 (a) and (b) show the pulse patterns for legs A and B of the Figure 8. Characteristics of the Harmonic Frequency Components U-phase of the five-level VSI in Figure 5. The patterns were in the VSI Output Power with the Fixed Pulse Pattern with Respect prepared to eliminate low frequency components, as described to the Motor Frequency. above. The same pulse pattern is applied to both legs but the timing is shifted. Because the output voltage shape is the same Though the asynchronous pulse pattern operation contains a good as the pulse pattern, each leg outputs three voltage levels. Figure number of harmonic components, this problem can be overcome by 6 (c) shows the U-phase output voltage with five output voltage using the fixed pulse pattern obtained in this study. In these figures, levels, which can be obtained by substituting the output voltage the red line indicates that the motor frequency behavior corresponds of leg B with that of leg A. Since the substitution is a linear to the harmonic frequency contained in the VSI output power. operation, the voltage shape does not contain low frequency Generally, long mechanical structures like the compressor trains components since the original voltage shape of each leg does have the lowest resonance frequency of around 20 Hz or less. Thus, not contain them. Figure 6 (d) shows the V-phase output the harmonics of this frequency range should be focused, as indicated voltage. The V-phase output voltage shape is the same as the by the rectangular region in Figure 7 and Figure 8. For example, in U-phase voltage with a 120 degree phase shift to produce the Figure 7, the motor frequency around 42.5 Hz indicates many low symmetrical three-phase output voltage from the VSI. Figure 6 order harmonics at frequencies from 0 Hz to 20 Hz in the inverter (e) indicates the line-to-line output voltage between the U-phase output power. However, the low order harmonics at these frequencies and the V-phase, showing nine voltage levels (Tsukakoshi, et al., are almost completely eliminated in Figure 8 by employing the 2009). Again, the synthesized line-to-line voltage does not proposed fixed pulse pattern. For other motor frequency, Figure 7 contain low frequency components. indicates many low frequency harmonics. On the other hand, Figure NOVEL TORQUE RIPPLE MINIMIZATION CONTROL FOR 25MW VARIABLE 197 SPEED DRIVE SYSTEM FED BY MULTILEVEL VOLTAGE SOURCE INVERTER 8 indicates very few low frequency harmonics. The higher order Configuration of the Drive System harmonics do not negatively influence the motor and drive system. (Five-Level GCT VSI) Experimental results obtained using a fixed pulse pattern will be shown later in the “RESULTS AND EVALUATION” section. The five-level GCT VSI system is configured with three pairs of single-phase three-level GCT inverters that operate as a THE TEST SYSTEM OF ACTUAL three-phase five-level inverter. The main circuit unit and main DIMENSIONS RATED AT 30 MVA circuit board can be interconnected with other drive systems. The system has been developed into a 30 MVA inverter that Test Setup produces a 7.2 kV output voltage. Because the capacity of a single-bank five-level GCT inverter is 30 MVA, it is possible to Figure 9 shows a photograph of the test setup rated at 30 MVA, obtain 120 MVA by connecting four banks together. When applied used for system performance evaluation. Tests conducted under to a compressor, it is not necessary for the variable frequency drive actual operating conditions are essential for evaluating the (VFD) to have a power regeneration ability. Therefore, a diode performance of the five-level inverter drive system controlled by converter circuit is introduced to achieve a smaller footprint and the proposed fixed pulse pattern. Figure 9 shows the photograph of lower cost. the test setup rated at 30 MVA for this purpose. A five-level GCT Figure 11(a) shows the structure of a prototype five-level GCT VSI rated at 30 MVA and a two-pole motor rated at 25 MW were inverter. Figure 11(b) shows the configuration of the main power designed and manufactured for the evaluation. The details of the block in a five-level GCT inverter. It was developed with main VSI and the motor will be explained in the following sections. The switching devices, such as GCTs, freewheel diodes connected in VSI output voltage harmonics, torque ripple and so on measured in parallel and coupling diodes. The specifications of the proposed the test setup are described in the following sections. Figure 9 also inverter are given in Table 1. shows the load, constructed of a generator and inverters.

Figure 9. Test Setup for System Performance.

Figure 10 shows a schematic diagram of the test setup. The five-level VSI powers the motor that is connected to the generator through a gearbox. The five-level GCT VSI controls the motor speed. The torque of the load, namely, the generator is controlled by the IEGT VSIs (Ichikawa, et al., 2004) to simulate the actual field operation up to full load condition. The feature of the test setup is the power regeneration. The electrical power taken from the generator Figure 11. (a) Outline of Five-Level GCT Inverter (Prototype) and passes through the IEGT VSIs and returns to the power system. This (b) GCT Power Unit. technique reduces the power consumption even for the full 30 MVA test, supplying only the make up power from the electrical losses, Table 1. Specification of Five-Level GCT Inverter. enabling a test under actual operation conditions (Tsukakoshi, et al., 2005). The generator for the load is rated at 25 MW and is of the synchronous type with four poles. The four parallel IEGT VSIs handle 32 MVA and 25 MW. A speed gear with a ratio 2:1 is connected between the motor and the generator to match the speeds.

Figure 10. Schematic Diagram of 30 MVA VSI Test System. 198 PROCEEDINGS OF THE THIRTY-NINTH TURBOMACHINERY SYMPOSIUM • 2010

Configuration of the Electric Motor Shaft Torque Measurement Figure 12 shows a cross-sectional diagram of the motor. Figure 15 shows the shaft torque measurement point in the test Though the basic configuration is the same as a two-pole turbine system. Torsional distortion of a high speed flexible joint was generator, it is necessary to consider the following matters measured by using a strain gauge and telemeter, and the shaft comparing to a generator: torque was calculated. • Speed fluctuation • Torque ripple • Harmonic losses • Surge voltage

Figure 15. Shaft Torque Measurement Point. Torsional Vibration Analysis of the Motor Method of Analysis Torsional vibration analysis was performed to examine the torque ripple due to the shaft, especially to analyze the strength of the elastic joint. Figure 16 shows an analytical model for the test system with a four-pole generator acting as a load connected through the gear. As the motor rotor is long in the axial direction, it is divided into 10 of inertia by considering the torsional modes of the rotor. The analytical model is divided into 24 mass points between the Figure 12. Cross-sectional Diagram of the Motor. motor and gear and between the gear and generator. Vibration analysis was performed using a commercially available technical Figure 13 shows a diagram of an elastically-supported stator. It computing software with this model. The lowest torsional vibration is designed in such a way that the 2n-element (electromagnetic in which the motor and generator are set in a reverse phase vibration oscillation at twice the power supply frequency) of the stator is not mode was 17.1 Hz. In addition, the torsional natural frequency was transmitted through the frame. 67.4 Hz and 200 Hz at an operating frequency of 200 Hz.

Figure 16. Torsional Vibration Analysis Model.

Figure 17 shows the results of the torsional inherence mode for 17.1 Hz, 67.4 Hz and 200 Hz. The results show the distribution of Figure 13. Diagram of Stator. the amplitude of vibration. For low frequencies, the normalized modal vector increases gradually with increasing number of inertia. Figure 14 shows a diagram of a rotor. It is configured with a reverse phase excitation system so that it is possible to startup from zero speed. The soundness of each part in terms of endurance strength was confirmed by repeatedly applying centrifugal force and fretting the mating part by torque ripples.

Figure 14. Diagram of Rotor. Figure 17. Torsional Inherence Mode. NOVEL TORQUE RIPPLE MINIMIZATION CONTROL FOR 25MW VARIABLE 199 SPEED DRIVE SYSTEM FED BY MULTILEVEL VOLTAGE SOURCE INVERTER Test Condition The 2n-element of the output frequency of the drive equipment The measurements were conducted assuming a rated output is the main cause of vibration, and no mechanical excitation is found frequency of 60 Hz by varying the load from zero to the maximum for other low order elements. However, a very small excitation occurs (100 percent). With the same load conditions, the authors also because of the influence of 2n-element, which can be neglected. conducted measurements at 70 percent of the rated output Figures 21 and 22 show the results of shaft torque ripple and frequency (42 Hz). Detailed results are shown in the next section. frequency analysis at the rated speed (3600 rotations per min) with 100 percent load. From Figure 21, the maximum shaft torque ripple RESULTS AND EVALUATION is 7.87 kN-m (about 5 percent) by assuming zero as the middle point. The influence of coupling and the axial strength is small. Figure 22 Inverter Voltage Waveform and FFT Analysis shows that the maximum shaft torque is 2.5 percent at 17 Hz. Figure 18 shows the test results of U-V line-to-line no-load voltage waveform for 60 Hz five-pulse operation. The fast Fourier transform (FFT) analysis of these test results is shown in Figure 19.

Figure 21. Waveform of Shaft Torque Ripple (3600 RPM, 100 Percent Load).

Figure 18. Five-Pulse 60 Hz Line-to-Line Voltage (U-V).

Figure 22. Frequency Analysis (3600 RPM, 100 Percent Load).

Figure 23 shows the relation between shaft torque ripple and load. Here, the element of the shaft torque ripple is considered to be the first-order specific frequency element. From the result, the shaft torque ripple increases in proportion to the load. However, the Figure 19. Comparison of Theoretical and Experimental Values for torque ripple is smaller because the fixed pulse pattern is employed. Five-Pulse No-Load Voltage by FFT Analysis.

In Figure 19, the theoretical values and the test results of zeroized low order harmonics by FFT analysis are compared showing that both values are approximately the same. The cancellation of the low order harmonics using only the inverter is one of the superior VSI characteristics realized by this advanced control method.

Results of Torsional Vibration Analysis of the Motor Figure 20 shows a three-dimensional illustration of the shaft torque at 5 percent load condition. From the figure, it is clear that torque ripple on the shaft is caused at the specific frequencies of Figure 23. Relation Between Shaft Torque Ripple and Load. 17.1 Hz and 67.4 Hz regardless of the rotational speed. When the 2n-element corresponds to those specific frequencies, the shaft Torque ripple from the load as a generator and three-level torque ripple becomes maximum. inverters are included in the test results. From these results, it is possible to calculate the torque ripple for a five-level inverter and the motor; it was calculated to be less than 1.5 percent. Figure 24 shows the test results for shaft torque ripple and frequency response.

Figure 20. Three-Dimensional Illustration of the Shaft Torque Ripple. Figure 24. Test Results for Shaft Torque Ripple and Frequency Response. 200 PROCEEDINGS OF THE THIRTY-NINTH TURBOMACHINERY SYMPOSIUM • 2010

Here, the generated torque ripple was 1.5 percent, the modal confirmed through these experiments. In this study, the authors damping ratio was 1 percent, and the number of torque generating achieved satisfactory results from the experimental tests, and they points was 6 to 16 for motor excitation and 24 for generator excitation. expect that their findings will be implemented in practical systems The measured shaft torque ripple was lower than the frequency in the near future. response curve for generator excitation shown by the black points in Figure 24. Considering the motor side inverter as a five-level REFERENCES inverter, the torque ripple can be calculated from the above values. Kocur, J. A., Jr., Corcoran, J. P., 2008, “VFD Induced Coupling The result gives a low-frequency torque ripple of less than 1.5 percent. Failure,” Thirty-Seventh Turbomachinery Symposium, Case In Figure 24, the red line indicates the estimated generator Study #09, Turbomachinery Laboratory, Texas A&M University, excitation line. When the mechanical excitation occurs by the College Station, Texas. generator, the frequency versus torque response characteristic can be estimated according to the model of Figure 16. On the other Ichikawa, K., Tsukakoshi, M., and Nakajima, R., 2004, “Higher hand, the response by the motor when mechanical excitation is Efficiency Three-Level Inverter Employing IEGTs,” Applied performed is shown in the blue line. The torque ripple in the Power Electronics Conference and Exposition, Anaheim, excitation source was calculated to be 1.5 percent and the modal California, 3, pp. 1663-1668. damping constant was calculated to be 1 percent. In Figure 16, a Ikonen, M., Laakkonen, O., and Kettunen, M., 2005, “Two-Level total of 24 mass points was used for the generator and excitation of and Three-Level Converter Comparison in Wind Power these mass points by the generator was treated as the excitation Application,” Department of Electrical Engineering, Lappeenranta source in the calculation. Moreover, the number of mass points for University of Technology, FI-53851, Lappeenranta, Finland, p. 3. the motor was from 6 to 16, and excitation of these mass points by the motor was treated as the excitation source in the calculation. Kita, M., Hataya, T., and Tokimasa, Y., 2007, “Study of a Rotor The torque ripple measurements conducted under various test Dynamic Analysis Method That Considers Torsional and conditions are plotted in Figure 24. From the results, most of the Lateral Coupled Vibrations in Compressor Trains with a points are located under the blue line. Some of these are presumed Gearbox,” Proceedings of the Thirty-Sixth Turbomachinery to be a ripple generated from the generator, which imitates the load Symposium, Turbomachinery Laboratory, Texas A&M though it is above the blue line, and there are measurements near University, College Station, Texas, pp. 31-38. the red line. Some are above the blue line, which is close to the red Shimakawa, T. and Kojo, T., 2007, “The Torsional Torque line. The generated torque ripple is caused by the generator Fluctuations of a Compressor Train with a Vector Control modeled as the load. Therefore, from these test results, the PWM Inverter,” Thirty-Sixth Turbomachinery Symposium, generated torque ripples in the motor and the five-level inverter are Case Study #01, Turbomachinery Laboratory, Texas A&M thought to be only the elements below the blue line. If there is no University, College Station, Texas. excitation from the load side, it should be possible to obtain a Tsukakoshi, M., Al Mamun, M., Hashimura, K., and Hosoda, H., smaller torque ripple of less than 1.5 percent. 2009, “Performance Evaluation of a Large Capacity VSD CONCLUSION System for Oil and Gas Industry,” 2009 IEEE Energy Conversion Congress and Exposition, San Jose, California, A new method of torque minimization in VSI systems that is pp. 3485-3492. suitable for large compressor applications is proposed. Besides its technical advantages, the proposed system, when used as the prime Tsukakoshi, M., Mukunoki, M., and Nakamura, R., 2005, “High mover for a large compressor train, will result in significant Performance IEGT Inverter for Main Drives in the Steel financial benefits for the end user. In this study, a strategy for how Industry,” IPEC2005, Nigata, Japan, S15-2. to avoid mechanical resonance by the synchronous control method ACKNOWLEDGEMENTS is described. The effectiveness of the proposed method was made possible by the use of a fixed pulse pattern that was synchronized The authors thank the Motor Department of Toshiba with the operating frequency of the drive system. The authors Mitsubishi-Electric Industrial Systems Corporation, for providing succeeded in experimentally evaluating the system prior to its use motor test results, including permission to reproduce them here. in the field by conducting a challenging task, which involved an The contributions to this project from many persons, all of whom experiment on a large system. Reduced torque ripple was cannot be listed here, are highly appreciated.